Ideally, the windows of a vehicle are clean and unobstructed so that a vehicle operator may safely and comfortably view the environment surrounding the vehicle. It is desirable to remove moisture and debris which is deposited on the windows so that the vehicle operator's view remains clear. Windshield wiper assemblies for wiping moisture and debris from the window of a vehicle have been long known in the art. These windshield wiper assemblies have generally included an elongated blade element which is supported by some type of support body. The blade element typically includes an elongated resilient blade or squeegee and an elongated blade retainer for supporting the blade. The blade includes a lower portion with a wiping edge for contacting and wiping the window. The wiper assembly's support body is mounted to an arm or lever which cooperates with the support body to move the blade element across the window of a vehicle thereby removing moisture and debris from the window surface. Ideally, the wiper blade assembly completely and consistently wipes the surface of the window, is functional and durable under a variety of conditions, and is low in complexity, weight, and cost.
A common problem with currently existing wiper assemblies is that the window is not completely and uniformly wiped by the blade element. Instead, streaking occurs wherein some portions of the window are wiped clear while others are incompletely wiped. Streaking generally occurs because the contact force between the blade element and the window is not uniform along the length of the blade element. Where the contact force between a portion of the blade element and the window is too low, that portion of the element does not effectively wipe the area of the window over which it passes. To prevent streaking, it is desirable to provide a wiper assembly which supports the blade element in such a way that the contact force between the blade element and the window is nearly uniform along the entire length of the element.
In the early days of motorized vehicles, windows were substantially flat or planar. In this situation, a blade element could be uniformly loaded along its length by a sufficiently rigid support body which held the blade element in a straight line parallel to the planar window surface. As vehicle design has evolved, glass surfaces have become non-planar to accommodate the demands of enhanced styling and improved aerodynamics. This trend has become especially pronounced in the last few years as window, and especially windshield, surfaces of automobiles have increased substantially in size. In the majority of modem vehicles, the windshield and other window surfaces are not only curved, but the curvature of the window surface varies depending upon the location measured. Therefore, a wiper assembly designed to wipe a modem curved glass surface must be capable of conforming to the variety of curvatures which will be encountered as the wiper assembly moves across the window surface. In addition, the wiper assemblies must be long enough to wipe the majority of the ever-increasing window surface area. Therefore, it is desirable to have a wiper assembly which is capable of dynamically conforming a blade element to a variety of curved glass surfaces while at the same time maintaining nearly uniform contact force between the element and the glass surface.
Another factor contributing to wiping quality is the torsional stability of the blade element. As a blade element of a wiper assembly is moved across a glass surface, the blade experiences significant lateral force at its wiping edge which resists the movement of the blade element across the glass surface. This force creates a twisting or torsional force in the blade element which, if unresisted, will cause the blade element to twist until the wiping edge is no longer held in contact with the glass surface. Ideally, the blade element is held such that the resilient blade is approximately perpendicular to the glass surface when the blade is not moving or loaded. As the blade element is moved laterally across the glass, the torsional forces cause the resilient blade to flex a small amount such that one side of the wiping edge contacts the glass. When the blade element moves in the opposite direction, the resilient blade deforms or "flips" in the opposite direction exposing the opposite side of the wiping edge. Modem blade elements are designed such that the amount of flip or deformation of the resilient blade is controlled so that optimal wiping can occur. The design of the blade element requires that the retaining portion of the blade element and the wiper assembly resist torsional forces so that the only significant deformation in the blade is due to the flex of the resilient blade itself rather than deformation of the entire assembly.
Besides trying to allow a blade element to dynamically conform to a curved glass surface while maintaining uniform contact force and resisting torsional forces, a wiper assembly must also be capable of functioning under a variety of changing conditions. One particularly challenging condition is the presence of ice, slush, or snow on the wiper assembly. This is generally referred to as "icing" of the wiper assembly. To prevent icing from interfering with the function of the wiper assembly, it is desirable to minimize the number of exposed joints, pivots, or moving parts since exposed parts are likely to collect snow or ice. When ice accumulates around or in a joint or moving part, the movement of the joint or part may be restricted thereby preventing the wiper assembly from functioning properly. Many modem windshield assembly designs include a large number of pivots and moveable links and are therefore highly prone to icing. In these situations, the typical solution is to cover the moveable parts with a flexible covering or "boot" so that ice and snow cannot accumulate between the moveable parts. However, this flexible covering adds cost and weight to the wiper assembly and may create an unpleasant physical appearance or reduce aerodynamic performance.
Because wiper assemblies are located on the exterior of a vehicle, they are often exposed to high speed wind. The wind force exerted on the wiper assembly may attempt to lift the wiper assembly out of contact with the vehicle window thereby reducing the contact force between the blade element and the window. This in turn adversely affects the performance of the wiper assembly. As air flows over the wiper assembly it may also create wind noise which is distracting and unpleasant for the vehicle's operator. It is desirable for a windshield wiper assembly to be designed such that the effects of wind lift and wind noise are kept to a minimum. Generally, the greater the cross-section, complexity, and number of structural members comprising the wiper assembly, the greater the tendency towards wind lift and wind noise. Therefore, it is desirable to minimize the cross-section, complexity, and number of structural elements required.
Currently available wiper assembly designs fall short of providing all of the desired features of an ideal wiper assembly. The most common currently available wiper assembly includes a blade element having an elongated elastomeric blade held in a plastic and/or metal blade retainer. The blade retainer is in turn supported at numerous points by a metal support body. The support body includes a variety of links and pivots interconnecting the retainer support points so that the blade element has a limited ability to conform to a curved glass surface. The support body is supported on a wiper arm which is biased toward the vehicle window so that force is transmitted through the support body to the blade element through the plurality of support points. This approach has several drawbacks. The use of multiple links and pivots is complicated and costly, and creates compliance in the assembly. Compliance may reduce the stability of the blade retainer and cause noise. This compliance will tend to increase as the assembly wears. In terms of wiping quality, the use of discrete support points to support the blade element leads to nonuniform contact force between the blade element and the window. Portions of the blade element immediately adjacent the support points are more heavily loaded than those portions of the blade element that are located away from or between the support points. Therefore, the contact force between the blade element and the window is nonuniform along the length of the blade element. The nonuniformity can be reduced somewhat by increasing the number of the support points. However, as the number of contact points is increased, the cost and complexity of the wiper assembly is also increased. Increasing the number of support points also requires an increase in the number of links and pivots. This in turn leads to additional compliance in the assembly and increases the assembly's susceptibility to icing, wind lift, noise, and wear. The nonuniformity of contact force can also be reduced somewhat by increasing the stiffness of the retainer portion of the blade element. This helps to distribute the load applied at the support points to the rest of the blade element. However, increasing the stiffness of the retainer portion adversely affects the ability of the wiper assembly to conform to a curved glass surface.
Another approach to providing a more uniform contact force between a blade element and a window is to support the blade element in tension using a minimum of support points. By supporting the blade element in tension at its ends, the contact force between the blade element and window is nearly uniform and independent of the curvature of the window. As the tensioned blade element conforms to the glass surface, a component of the tensioning force is directed towards the glass and tends to create a uniform contact force between the blade element and the glass. The blade element is capable of dynamically conforming to a variety of glass curvatures. It also can easily conform to a glass surface where curvature is greater toward one end of the blade. By eliminating intermediate support points, point loads are eliminated thereby preventing portions of the blade element from being more heavily loaded than the surrounding portions. There have been several attempts to provide a wiper assembly which supports a blade element in tension, but each of these attempts has drawbacks.
U.S. Pat. No. 2,167,207 to Horton discloses a wiper assembly which places a blade element in tension. The support body has an integral spring which biases the support body to tension the blade element. The complicated support body leads to a heavy and expensive wiper assembly which would be prone to icing. Torsional forces may be resisted by supporting the element in a slotted tube. However, this form of support stiffens the blade element allowing it to conform only to minor irregularities in the glass surface.
U.S. Pat. No. 2,659,097 to Morton discloses a wiper assembly which creates tension in the blade element using a spring that is integral with the blade element. Tension is created in the blade element by attaching opposite ends of the integral spring to a metal support body. This approach allows the blade element to conform to a variety of surfaces and maintains reasonably uniform contact pressure between the blade element and the glass surface. However, his approach to creating tension also places the elastomeric blade in tension which creates stress in the blade which adversely affects its performance and longevity. The use of an integral spring also complicates the manufacturing of the blade element and the installation of the blade element into the support body. The wiper assembly relies on the integral spring to resist torsional forces. Also, the use of a metal support body having sufficient strength to resist the integral spring causes the wiper assembly to be heavy.
U.S. Pat. No. 3,132,367 to Wise discloses a wiper assembly which places the blade element in tension by supporting the blade element at both of its ends using a spring steel support body. This design lacks any intermediate support of the blade element to prevent twisting of the blade element. Instead, twisting is prevented by a retainer portion of the blade element being made of steel. In order for this retainer portion to successfully resist twisting of the blade element, it must be stiff enough such that it will impede the ability of the blade element to conform to a curved glass surface. Therefore, while the design may be able to conform to a slightly curved surface, it will be unable to conform to the more radically curved surfaces of modem vehicle windows.
U.S. Pat. No. 3,392,415 to Shipman discloses a wiper assembly which places the blade element in tension using a support body with a telescoping end section and a spring which biases the support body to a longer length. The telescoping end section adds cost, complexity and weight to the support body and also is prone to interference from icing. The design also lacks any type of intermediate torsional support.
U.S. Pat. No. 3,874,020 discloses a wiper assembly which includes a support body having a central pivot. The support body supports the blade element at its ends such that when a downward force is placed at the pivot, the support body creates tension in the blade element. This design relies on the downward force created by the wiper arm to create tension in the blade element. The design also includes a metal insert in the blade element which creates additional downward force in the center of the span of the blade element. This piece complicates the manufacture of the blade element and increases its cost and complexity. The design lacks any type of intermediate torsional support.